Spawns work. Just spawned 40 Blenheims at once
Maybe you did not start the battle or selected side first?
I do not recommend using splineroads or many small objects in an online mission (with spawns) because they increase loading time, reduce FPS and increase lag. You will have to remove all these sandboxes and sirens for online use I am afraid.
It is there for offline and future online use, maybe in 2-3 years when we have better hardware.